T lymphocytes must be replaced continually throughout life, a process that is the primary function of the thymus. This process initiates with a very small number of progenitors that periodically home to the thymus from the bone marrow. Following this, microenvironmental signals specific to the thymus induce multiple changes in these cells, leading ultimately to the production of functional T lymphocytes. These changes include a million-fold proliferative expansion, specification of T lineage fate, and selection based on antigen receptor specificity. In the two previous project periods, we have defined some of the thymus-specific conditions that support the early lymphopoietic phases of this process (i.e., proliferation and lineage commitment). In particular, we have identified tissue compartments within the thymus where specific developmental transitions take place, and have identified some of the signals that take place within these compartments, as well as some of the interactions that facilitate directional migration between them. The goals of the current application are 1) to characterize the roles several additional thymus-derived microenvironmental signals, as revealed by our previous studies, in early T cell lineage commitment and/or proliferation; 2) to determine the roles of a newly identified and tightly regulated transcription factor in control of the early lymphopoietic process; and 3) to define tissue-specific signals that induce and support the major developmental transition from early (lineage double-negative) to intermediate (lineage double-positive)stages of development. These studies are aimed at defining the processes by which the thymus supports normal T cell production, and in addition to an understanding of the basic biology of this process, it is anticipated that such knowledge may be used in the treatment of T cell immunodeficiencies, especially secondary immunodeficiencies associated with aging.